Fuels cells are widely used devices design to generate electric power. Principle uses of fuel cells include automobiles and power generation in remote areas. Numerous high cost materials are employed in the construction of fuel cells including large amounts of platinum metal and specialized polymer electrolyte membranes. Fuel cells have a limited life span due to wear and tear on the components including changes to the performance of the polymer electrolyte membrane that occurs over time. Recovery of platinum from used fuel cells represents a large value that would otherwise go to waste. Efficient methods of recycling reduce the cost of employing fuel cells allowing for more widespread use.
Fuel cells convert a fuel and an oxidizing agent into electricity, heat, and water. Fuel cells are composed of a polymer electrolyte membrane sandwiched between an anode and a cathode, and the polymer electrolyte membrane also serves to keep the fuel and oxidizing agent locally separated. The polymer electrolyte membrane is selectively permeable and non-conductive, for example, the polymer electrolyte membrane is permeable only to hydrogen ions in a hydrogen/oxygen fuel cell. The reactions at the cathode and anode may be summarized as follows:H2→2H++2e−(Anode)→½O2+2e−→H2O(Cathode)  (1)
The polymer electrolyte membrane, anode, and cathode are further sandwiched between two gas diffusion layers forming five layers in total, referred to as a membrane electrode assembly. The gas diffusion layers are formed from porous, fibrous carbon fibers allowing for gaseous reactants and products to diffuse toward or away from the anode and cathode. The anode and cathode are formed from platinum-containing electrode catalyst layers that are deposited on the surface of either the gas diffusion layers or the polymer electrolyte membrane. Electrode catalyst layers deposited on the gas diffusion layer are known as gas diffusion electrodes, and those having the electrode catalyst layers deposited on the polymer electrolyte membrane are known as catalyst coated membranes. The terms gas diffusion electrode assembly and catalyst coated membrane assembly, respectively, refer to membrane electrode assemblies having the respective type of electrode catalyst layers.
The electrode catalyst layers typically contain precious metals as active catalytic components in addition to other components including conductive supporting material. For example, 0.5-4 mg/cm2 of platinum can be applied to the electrodes in the form of an ink or using complex chemical procedures. Platinum is a significant cost in the fabrication of a fuel cell.
The bulk of the membrane electrode assembly is carbon-based; therefore, a standard method to recycle precious metals, including platinum, involves a combustion step to remove carbon material. However, membrane electrode assemblies have high fluorine content due to polytetrafluoroethylene (PTFE) impregnated on the carbon fibers and from common polymer electrolyte membrane materials, such as Nafion® (DuPont Co., Wilmington, Del.), which results in a large, undesirable discharge of HF upon combustion. Removal of HF gas involves scrubbing and dedicated equipment that can withstand the corrosive nature of HF gas. Isolating the combustion from existing infrastructure is recommended to localize maintenance needs caused by the effects of HF gas.
Alternate methods of fuel-cell membrane electrode assembly recycling can withstand improvement when attempting to process both catalyst coated membrane assemblies and gas diffusion electrode assemblies. Fuel cell recyclers cannot easily determine if a membrane electrode assembly is a catalyst coated membrane assembly or a gas diffusion electrode assembly unit, which can potentially result in a loss of precious metal recovery if the wrong process is used. Known methods, including combustion, also result in loss of ruthenium, often present as a catalyst in addition to platinum, and polymer electrolyte membrane material that can potentially be economically recycled in a downstream process after precious metal recovery.